1. Field of the Invention
The present invention relates to a method of controlling a supply of electric power to an electric load and a circuit thereof. More particularly, the present invention relates to a method which controls a phase of AC voltage being supplied to the electric load at every cycle of the AC voltage in order to fade out the power consumption of electric energy of the load for a predetermined time period, and a circuit which performs the method.
2. Prior Art
Generally, an electric device, such as an electric lamp, an electric heater or the like, is powered on or off by an on/off switch. In case of the electric lamp, a variable resistor is used for controlling the brightness of lamp, and a power switch is used for turning on the lamp, such that a user can reduce the brightness of the lamp or turn off the lamp before sleeping.
On the other hand, the facility and utility of lamps have been improved as a lamp having a function of a timer. The function of timer of the lamp may be used in sleeping. That is, when a user sets a turn-off time of the lamp, the needless lighting of the lamp is prevented.
FIG. 1 is a view for showing a circuit of a conventional power control circuit for an electric lamp.
Referring to FIG. 1, the circuit includes a time setting part 71, a reference voltage generation part 72, a comparing part 70, and a driving part 73. The time setting part 71 includes a resistor and a capacitor for setting a time period of supplying power to a lamp LP. The time period is achieved by a time constant of the resistor and the capacitor. The reference voltage generation part 72 generates a reference voltage which is used for determining whether a discharging voltage of the capacitor is greater than the reference voltage. The comparing part 70 compares the discharging voltage with the reference voltage, and outputs a voltage. The driving part 73 turns on or off the lamp LP according to the voltage based on the comparison result of the comparing part 70.
Further, the reference voltage generation part 72 includes resistors R2 and R3 and a diode D.
The time setting part 71 includes the resistor R1, the capacitor C1, and a variable resistor VR. The time constant of the time setting part 71 is adjusted by the variable resistor VR.
The comparing part 70 includes an operational amplifier OP and two resistors R4 and R5 for comparing the reference voltage with the discharging voltage.
The driving part 73 is a switching device such as a triac or the like.
Hereinafter, the operation of the circuit will be described.
When the driving part 73 is turned off, the AC source AC is not applied to the lamp LP. When the switch SW is switched on by a user, the capacitor is charged with electric energy which is supplied through the resistor R1 from the DC source Vcc.
At the same time, the operational amplifier OP outputs a high level of voltage to the triac T of the driving part 73 because of a voltage difference between the two terminals of resistor R2, such that the AC power is supplied from the AC source to the lamp LP.
When the switch SW is switched off, the capacitor starts to discharge the charged energy thereof to the variable resistor VR. However, although the switch SW is switched off, the operational amplifier OP continuously outputs the voltage of high level until the value of the discharging voltage is less than that of the reference voltage.
As the discharging voltage of the capacitor C is gradually decreased, the difference between the reference and discharging voltages is decreased, too. Therefore, by varying the resistance value of the variable resistor VR, the time constant of the time setting part 71 is determined, such that the light time of the lamp LP is adjustable.
As the voltage of the non-inverting terminal is decreased, the voltage value is less than that of the reference voltage, such that the operational amplifier OP outputs a low level voltage. As the result of outputting the low level voltage, the gate voltage of the triac T drops down to zero, such that the lamp LP is turned off.
Therefore, the circuit controls the power supply of the lamp as one example of electric loads by directly either connecting or disconnecting the AC source with the lamp, or turning off the lamp when a time set by a timer passes since the lamp is turned on.
However, the control circuit has problems that the lamp is abruptly turned off so that the eye of a user may be stimulated by the abrupt variation of the brightness, and the power consumption of the circuit is much more.